1. Field
The present invention generally relates to structures for the alignment of the components of a mechanical assembly, such as an optical package. More specifically, some embodiments of the present invention relate to optical packages and the alignment of components thereof such that an output beam of a laser is positioned upon a waveguide input of a wavelength conversion device.
2. Technical Background
In many applications, there is a need for extremely accurate mechanical connection between components of an assembly. For example, accurate optical coupling is required in the assembly of component parts of a frequency doubled green laser apparatus or system. In such an application, a nonlinear optical crystal, such as a Mg—O doped periodically poled lithium niobate (PPLN) crystal, is used to convert the infrared light emission of a laser into visible green light. Both the diode laser and nonlinear optical crystal use single mode waveguide structures to confine and guide the light energy. In such a green laser application, there is a need for the components of the assembly to be maintained in rigid alignment such that the output beam of the laser is precisely aligned with the very small waveguide input that is located on an input face of the waveguide crystal. Waveguide optical mode field diameters of typical second harmonic generating (SHG) crystals, such PPLN crystals, can be in the range of a few microns. As a result, the present inventors have recognized that it can be very challenging to properly align and focus the output beam from the laser diode with the waveguide of the SHG crystal, particularly during assembly of the optical package.
Tolerances on the alignment of the laser and nonlinear crystal waveguides may be between 300 nm and 500 nm (for 5% degradation in coupling) in the plane perpendicular to the optical axis. The tolerance along the direction of the optical axis may be significantly looser, between about 3 μm and 4 μm. Therefore, the slightest misalignment between the laser output beam and the waveguide input may result in reduced coupling of the infrared energy and result in a loss of green output power.
Generally, there are two strategies to aligning the components in the green laser assembly: a passive alignment approach and an active alignment approach. In the passive alignment approach, a permanent attachment technique, such as laser welding or UV cured adhesive, is utilized to achieve a rigid, accurate attachment between components of the green laser. With regard to laser welding, due to weld heating and stresses, post-weld part shifts occur and it is difficult to achieve assembly accuracy better than about 1 μm. The requirements of the green laser assembly require an order of magnitude better accuracy (positional accuracy on the order of 0.1 μm is needed). UV cured adhesives make achieving assembly accuracy of approximately 0.1 μm possible, but such adhesives are susceptible to swelling due to heat and humidity. The stability of the components relative to one another must be maintained to a few tenths of a micron over the lifetime of the laser and a wide range of environmental conditions (e.g., +10° C. to +60° C., up to 85% relative humidity).
In an active alignment approach, an adjustable active component is used to insure that the infrared energy from the laser is accurately aligned with the small input of the crystal waveguide. Because of this adjustability, the requirements for alignment of the various component parts of the device can be relaxed by an additional order of magnitude or so, allowing the components to be assembled to much more relaxed positional tolerances, on the order of tens or hundreds of microns. The active component or components may also be used to accommodate alignment changes during the life and operation of the laser. The downside of the active alignment approach is the active component itself. Typically, an active component is either a piezo-electric actuator or micro-electro-mechanical (MEMS) mirror device, which adds cost to the entire package, and reduces the overall reliability. Such devices can be susceptible to breakage during assembly, failures from environmental exposure, and shock induced damage.